Not applicable.
The present invention generally relates to the preparation of liquid hydrocarbons from natural gas/methane, oxygen and/or steam. In particular, the present invention relates to improved methods for preparing liquid hydrocarbons from improved feedstock streams.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling parts in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Reaction (1).
CH4+H2O⇄CO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. The steam reforming reaction is endothermic (the heat of reaction (1) is about 9 kcal/mol of methane), requiring the expenditure of large amounts of fuel to produce the necessary heat for the industrial scale process. Another drawback of steam reforming is that for many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
The catalytic partial oxidation (xe2x80x9cCPOXxe2x80x9d) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial or direct oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Reaction (2):
CH4+xc2xdO2⇄CO+2H2xe2x80x83xe2x80x83(2)
The H2:CO ratio for this reaction is more useful for the downstream conversion of syngas to chemicals such as methanol or other fuels than is the H2:CO ratio from steam reforming. In addition, the CPOX reaction is exothermic (xe2x88x928.5 kcal/mol), in contrast to the endothermic steam reforming reaction. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes than is possible in a conventional steam reforming process. All of these factors lower the cost for the conversion of methane or natural gas and make the CPOX reaction much more attractive for commercial use.
After syngas is obtained through the above-mentioned process, it is then converted to higher hydrocarbons (paraffins in the range of C5 to C20) by a variety of liquid hydrocarbon synthesis processes. One such process is via Fischer-Tropsch (FT) synthesis using a metal catalyst, through Reaction (3).
CO+2H2xe2x86x921/n(CnH2n)+H2Oxe2x80x83xe2x80x83(3)
There are primarily two broad types of catalyst used in FT synthesis: Fe-based catalysts and Co-based catalysts. The literature is replete with discussions of these catalysts and their varying compositions. Cobalt catalysts are generally considered a better match for the conversion of methane or natural gas derived syngas. For these catalysts, sulfur and oxygen are poisons that must be removed from the feedstock. It is known that sulfur will permanently deactivate a cobalt catalyst when present in concentrations of 50 ppb or greater. It has recently been discovered that oxygen can temporarily or even permanently deactivate a cobalt based catalyst when present even in very low amounts. The term xe2x80x9cnon-toxicxe2x80x9d will be used herein to describe a syngas stream having an oxygen concentration at or below a level that is acceptable for whatever application the syngas is to be used downstream, including catalytic FT synthesis.
The concentration of oxygen in syngas is typically determined by the reactive process used to derive the syngas. Traditional methods for producing synthesis gas, including steam methane reforming and auto-thermal reforming, are characterized by relatively long periods of reactant exposure to the catalyst beds. Long exposure times allow the reaction to consume any unconverted oxygen that may remain in the syngas.
Due to the commercial importance of syngas, there is a continuing effort to maximize the efficiency of syngas and liquid hydrocarbon productivity by developing new methods for preparing syngas. Smaller catalyst beds and shorter exposure times characterize some of these new methods. These new reactors are commonly referred to as short contact time reactors (SCTR). There are several advantages to short contact time reactors, i.e., increased productivity due to higher space velocities, smaller volumes of catalyst needed, smaller catalyst beds, etc.
In spite of the benefits, there is a greater opportunity for oxygen to pass through the reactor unconverted. One such opportunity for oxygen to pass through the reactor unconverted is due to the increased velocity of the gas through a thin fixed bed reactor. The traditional methods mentioned above generally have gas hourly space velocities (GHSV, the standard volume of gas flow through per volume of catalyst per hour) near 4,000 per hour, whereas the new SCTR designs can have GHSV as great as 1,000,000 per hour or higher. Oxygen breakthrough can result due to the millisecond residence time of the reactants. The higher space velocities also force gas to pass through xe2x80x9cshort cut channelsxe2x80x9d the catalyst bed or fractures in the insulation refractory before it can be exposed to the active catalyst resulting in increased concentrations of unconverted oxygen in the syngas product.
In addition, it is well known in the art that catalysts xe2x80x9cagexe2x80x9d with time and use. Aging occurs for a variety of reasons including coke deposition, poisoning, etc. The more aged a particular catalyst is the less efficient the catalyst is at initiating reaction, i.e., less activity it has. As the catalyst ages more oxygen may pass through the bed unconverted.
For at least these reasons, some oxygen is able to pass through a syngas reactor without being converted. This increase in the unconverted oxygen concentration can lead to a decrease in efficiency of the downstream Fischer-Tropsch process due to oxidation or poisoning of the catalysts. Hence, in natural-gas derived syngas, especially those obtained from short contact time selective oxidation processes, there exists up to 0.5(vol.) % oxygen, which can deactivate FT catalysts within several hours. Frequent regeneration of FT catalysts not only increases the difficulty of operation, but also significantly increases the associated costs.
Therefore, it is desired to decrease the oxygen concentration of the syngas by providing a method and apparatus for removing oxygen that remains in the syngas before the syngas is used in any downstream process, particularly a Fischer-Tropsch reaction.
The present invention is directed towards producing liquid hydrocarbons from a hydrocarbon containing gas and an oxygen containing gas or steam. The invention is identified as a process for converting methane to liquid hydrocarbons comprising (a) reacting hydrocarbon-containing gas, such as methane or natural gas with oxygen, air or some other oxygen source in a syngas reactor to produce syngas; (b) decreasing the amount unconverted oxygen in the syngas stream of step (a) to produce a non-toxic stream of syngas; and (c) reacting the non-toxic stream of syngas from step (b) in a synthesis reactor, i.e., Fischer-Tropsch reactor, to produce liquid hydrocarbons. Non-toxic streams of syngas are generally referred to as having preferably less than 1000 ppm, more preferably less than 100 ppm, and more preferably less than 10 ppm oxygen.
Catalyst compositions, apparatus and methods for the removal of oxygen are also identified as a means to achieve the present invention in accordance with the preferred embodiments. The oxygen removal process can be achieved by any means known in the art, however, the following techniques are considered to be the preferred embodiments of the present invention: selective catalytic reaction of oxygen and other gases, such as CO, H2 or CH4, as shown in Reactions (4), (5) and (6), respectively;
2CO+O2xe2x86x922CO2xe2x80x83xe2x80x83(4)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(5)
CH4+2O2xe2x86x922H2O+CO2xe2x80x83xe2x80x83(6)
oxygen selective membranes (with and without catalyst coatings); adsorption beds or columns; pressure-swing adsorption through physisorption on molecular sieves; Fischer-Tropsch wax non-catalytic or catalytic oxidation to produce oxygenates in F-T product stream; consuming the oxygen as a reactant in the functionalization of higher hydrocarbons, i.e., using the oxygen to convert hydrocarbons to alcohols, aldehydes, ketones, acids, etc.; liquid absorption with medium containing phenols, alcohols, and the like; using longer residence times in vessel downstream of the catalytic bed to combust oxygen (burn O2) through homogeneous reactions; methane oxidation via a secondary bed or layer in CoPOX reactor on a modified catalytic partial oxidation catalyst with greater O2 selectivity; any Redox reaction occurring in adsorption bed, membrane, production of metal oxides; homogeneous catalysis for hydrogen peroxide (H2O2) production; cryogenic separation of O2 from CO, CH4 and H2; biocatalytic reaction with cell-free enzymes such as but not limited to oxygenases, oxidoreductases, hydrogenases, or any combination thereof and in any form known in the art of biocatalysis such as but not limited to compounds immobilized, encapsulated, solubilized, purified, extracted from cells, as well as with whole organisms; respiration of organisms; biosorption; and any combination thereof.
Specific examples of these embodiments are disclosed herein. For example, metal and metal oxide catalysts are identified for the selective conversion of oxygen to carbon dioxide by reacting the oxygen and carbon monoxide (Reaction (4)) present in a gas stream such as a syngas product stream, a Fischer-Tropsch feedstock or other gas stream containing carbon monoxide.
According to the present invention, the preferred embodiment of the metal/metal oxide catalysts to remove unconverted-oxygen from these types of gas streams have the general formula xcex1AOx-xcex2BOy-xcex3COz, wherein:
A is one of the precious metals Rh, Ru, Pd, Pt, Au, Ag, Os or Ir or is a transition metal chosen from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Tc, Hf, Ta, W, Re, preferably Fe, Co, Ni, Mn, V or Mo or any combination of the above;
B is a rare earth metal La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y and Th, preferably La, Yb, Sm or Ce;
C is an element chosen from Group II (i.e., Be, Mg, Ca, Sr, Ba and Ra), III (i.e., B, Al, Ga, In, Tl) and IV (i.e., C, Si, Ge, Sn, Pb) elements of the Periodic Table of the Elements, preferably Mg, Al or Si;
O is oxygen;
xcex1, xcex2, xcex3 are the relative molar ratios of each metal oxide and xcex1=0-0.2; xcex2=0-0.5; xcex3=0.5-1; and
x, y, z are the numbers determined by the valence requirements of the metals A, B, and C, respectively. Their value can be zero when the corresponding metal stays in the metallic states.
In this general formula, if component A is in metallic form, this general formula can be presented as xcex1A-xcex2BOy-xcex3COz. Alternatively, the catalyst can take a general formula as xcex1AOx-xcex3COz when component B is not used. The codes, A, C, O, xcex1, xcex3, x, z, etc. have the same meaning as described above. Furthermore, if component A is in metallic form, this general formula becomes to xcex1A-xcex3COz.
With these catalysts, oxygen can be removed from syngas, a Fischer-Tropsch feedstock or other gas stream containing CO and oxygen in the temperature range of 20 to 600xc2x0 C., preferably 50 to 350xc2x0 C., more preferably 50 to 300xc2x0 C. The operative temperature range of the present invention is an advantage in that at least one embodiment allows the oxygen removal reactor to use the heat carried from the syngas reactor. This eliminates or at least greatly reduces the costs associated with any additional equipment or energy necessary to sustain the reaction.
In an alternative preferred embodiment, the concentration of oxygen is decreased in synthesis gas by flowing the synthesis gas along a membrane that is permeable to oxygen but not to synthesis gas. The synthesis gas is passed through an inner tube of selective membrane material. The inner tube is contained within a non-permeable outer tube such that an annulus is formed between the inner and outer tubes. A oxygen partial pressure differential is established between the inner tube and the annulus by either a pressure gradient or a concentration gradient across the membrane. This pressure differential between the inner tube and the annulus causes the oxygen to permeate through the inner tube toward the annulus.
Once separated from the synthesis gas, the oxygen can then be consumed in one of two reactions. The first is an oxidation reaction with hydrogen to form water according to reaction (5). The second is an oxidation reaction with carbon monoxide to form carbon dioxide according to reaction (4). In either reaction (4) or (5), the reaction can be driven by catalyst preferably dispersed on the outside surface of the inner tube. According to the present invention, metals that can be used to catalyze the hydrogen oxidation reaction include but are not limited to: Pt, Pd, Ni, Rh, Co, Fe, Au, Ag, Cu and Mn. The preferred metals are Pt, Pd, Rh, and Au. In addition, promoters such as rare earth metal oxides can be added to improve the catalytic activity of the catalyst. Other metal oxides can also be used as the active catalyst including: Co3O4, CuO, MnO2, NiO, Cr2O3, SnO2, Fe2O3, PbO and ZnO. The preferred oxides are Co3O4, CuO, MnO2 and mixtures thereof. Also according to the present invention, the following metals (platinum group metals such as Pt, Pd, Ir, Rh and Ru) and metal oxides (such as NiO, Cr2O3, ZnO and Fe2O3) can be used to catalyze the carbon monoxide oxidation reaction. The more preferred catalysts are the metal and/or metal oxides with or without promoters that can catalyze both the hydrogen and carbon monoxide reactions, such as Pt, Pd, Rh, NiO, Cr2O3, ZnO and Fe2O3 
In order to allow reaction (4) or (5) to occur, a gas that contains hydrogen or carbon monoxide, i.e., syngas, should be circulated through the annulus area to supply the reactant needed for the reaction with oxygen. Any unconverted oxygen passed directly into the annulus can also react on the catalyst on the outer surface of the inner tube. Thus, when the inner tube and annulus gas streams are combined into a final syngas stream the syngas should have an acceptable or non-toxic oxygen concentration, preferably less than 1000 ppm, more preferably less than 100 ppm, and more preferably less than 10 ppm.
According to this embodiment, oxygen can be removed from syngas, a Fischer-Tropsch feedstock or other gas stream containing CO and oxygen in the temperature range of 20 to 1000xc2x0 C., preferably 50 to 600xc2x0 C., and more preferably 50 to 300xc2x0 C.
In still another specific preferred embodiment, a reduction in the concentration of oxygen in synthesis gas is achieved by flowing the synthesis gas over an adsorption matrix that will selectively react to adsorb oxygen but not synthesis gas. As with the other embodiments, the syngas reactor and process can be any known in the art, i.e., partial oxidation, steam reforming, etc., because the syngas reaction and processes are not critical to the present invention. In the most preferred embodiment, the synthesis gas is passed through a packed bed of the selective adsorbent material. The adsorbent material is preferably prepared via incipient wetness impregnation on a support material followed by reduction to obtain the active oxide.
Metal and metal oxide adsorbents are identified for the selective adsorption of oxygen present in a gas stream such as a syngas product stream, a Fischer-Tropsch feedstock or other gas stream containing carbon monoxide. According to the present invention, the adsorbents to remove unconverted oxygen from these types of gas streams are preferably comprised of the transition metals and/or their oxides. In particular, the following metals are preferred: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb and Mo, more preferably Mn, Fe, Co, Ni and Cu, most preferably Mn and Co.
Depending on the type of reactor used, the oxygen-removal adsorbent can be produced in many different shapes, such as spheres, granules, cylinders, rings, extrudate, pellets etc. In the preferred embodiment, the catalyst size can range from {fraction (1/100)} of an inch to 1 inch, preferably from {fraction (1/50)} to xc2xd inch. Alternatively, the catalyst can be a monolith, foam, or honeycomb. In addition, the adsorbent support material can be any type of high surface area, porous and inert material. The preferred supports would include at least silica, alumina, zirconia, MgO, TiO2, and the like. The most preferred support is silica.
The packed bed is contained within a non-permeable housing such that a column is formed. In the most preferred embodiment, the oxygen is removed from the synthesis gas by the following reaction:
MnO+xc2xdO2xe2x86x92MnO2xe2x80x83xe2x80x83(7)
The overall process design preferably has more than one column with associated process equipment, such that when one column is completely saturated and no more oxygen adsorption can occur, the second column can brought on-line and the synthesis gas stream switched over. The oxygen can then be removed form the adsorbent in the first column, i.e., the saturated adsorbent bed can then be rejuvenated so that it can be used over and over again. The adsorbent is rejuvenated by the following reaction:
MnO2+H2xe2x86x92MnO+H2Oxe2x80x83xe2x80x83(8)
According to present invention, oxygen can be removed from syngas, a Fischer-Tropsch feedstock or other gas stream containing CO and oxygen in the temperature range of 20 to 100xc2x0 C. The reaction of equation (7) is an exothermic reaction and exhibits a heat of release in the range of about 100 to about 500 KJ per mole of O2 depending on the adsorbent composition used. The total heat release per hour is obviously dependent on the feed flow and the oxygen concentration within the gas. The rejuvenation reaction of the adsorbent material is an exothermic reaction and operates in the temperature range of 100-700xc2x0 C. Thus, according to the present invention, the temperature needed for the oxygen removal reaction can come directly from the heat carried with the syngas stream. This minimizes the need for any additional equipment and reduces capital costs. Appropriate insulation is enough to keep the reaction going to completion. However, heat may need to be provided for the rejuvenation reaction if the reactor is to be rejuvenated in the same location along the continuum between the syngas and Fischer-Tropsch reactors. The additional heat can be supplied from any means known in the art including passing a liquid or gas, i.e., water, steam or syngas, etc., over and around the outer surface of the adsorption columns. The preferred heating would be to use the heat from the syngas as it exits the syngas reactor. The syngas could then be treated as desired and passed through the oxygen removal reactor of the present invention.
Once the syngas stream has been exposed to the adsorbent bed, the oxygen is selectively removed from the syngas stream producing a non-toxic syngas stream. The produced non-toxic syngas can then be further treated or manipulated as needed and used as a Fischer-Tropsch or other downstream process feedstock. As with the other embodiments described herein, the Fischer-Tropsch reactor and process can be any technique known in the art, i.e., fixed or fluid beds, slurry bubble column(s), etc., because the Fischer-Tropsch reactions and processes are not critical to the present invention.
This invention further discloses a process comprising feeding a syngas stream with an oxygen concentration reduced to a non-toxic level to Fischer-Tropsch reactor, wherein the syngas stream is produced by partial oxidation of a hydrocarbon containing feed.
The oxygen removal reaction of the present invention does not have any significant side reactions such as methanation or carbon monoxide disproportionation. Thus, the hydrogen to carbon monoxide ratio, which may be valuable to subsequent synthesis reactions such as Fischer-Tropsch, is not significantly affected by the oxygen removal reactions.